improve self-connect error message (#5192)

mitmproxy/addons/proxyserver.py

@@ -140,6 +140,7 @@ async def refresh_server(self):
                 except OSError as e:
                     ctx.log.error(str(e))
                     return
+                # TODO: This is a bit confusing currently for `-p 0`.
                 addrs = {f"http://{human.format_address(s.getsockname())}" for s in self.server.sockets}
                 ctx.log.info(f"Proxy server listening at {' and '.join(addrs)}")
 
@@ -208,4 +209,7 @@ def server_connect(self, ctx: server_hooks.ServerConnectionHookData):
             ctx.server.address[0] in ("localhost", "127.0.0.1", "::1", self.options.listen_host)
         )
         if self_connect:
-            ctx.server.error = "Stopped mitmproxy from recursively connecting to itself."
+            ctx.server.error = (
+                "Request destination unknown. "
+                "Unable to figure out where this request should be forwarded to."
+            )

test/mitmproxy/addons/test_proxyserver.py

@@ -171,7 +171,7 @@ def test_self_connect():
         ps.server_connect(
             server_hooks.ServerConnectionHookData(server, client)
         )
-        assert server.error == "Stopped mitmproxy from recursively connecting to itself."
+        assert "Request destination unknown" in server.error
 
 
 def test_options():